1. Field of the Invention
The present invention generally relates to microstrip patch antennas. More particularly, the present invention relates to deriving dual-band performance from micro-strip patch antennas.
2. Related Art
Micro-strip patch antennas are popular because of their compact size, light weight, and low cost to fabricate. Their low profile and their ability to conform to the shape of an aircraft fuselage make these microstrip antennas particularly attractive for airborne satellite communications and navigation systems. Additionally, micro-strip patch antennas can easily be constructed using printed circuit technology. However, a major limitation is their relatively narrow bandwidth capability.
Many patch antenna applications, such as navigational, vehicular, wireless communication systems, and radar systems, require dual-band, or wideband, operation. The narrow bandwidth capability of conventional micro-strip antennas, however, limits their use in these applications. Since patch antennas are so desirable, however, significant research and development has been devoted to adapting these antennas to dual-band operation.
A common technique to obtain dual-band operation from a conventional micro-strip antenna is known as stacking. Stacking entails simply piling micro-strip antennas or radiators, each operating at different frequencies, on top of each other. Stacking conserves space in a transverse or lateral direction. In a stacked patch design, the second frequency is achieved by the lower patch radiator. This lower patch is larger in size compared to the upper patch and is tuned to the lower of the two desired frequencies. Unfortunately, however, the performance of the lower patch is degraded by blockage from the upper patch and by any other patches in close proximity thereto. And the gain and beam width of the bottom patch antenna radiator is often degraded by stacking.
Stacking also increases the vertical height of the antenna, making it unattractive for low-profile and conformal applications. An additional complication of stacking is the need for bonding the top and bottom patch antennas together with glue or some other bonding agent. This bonding further increases the overall cost and complexity of building the antenna.
A second technique used to derive dual-band performance from a conventional micro-strip patch is to provide slots in the antenna. Although a variety of slotted micro-strip antennas have been designed, most are limited by their polarization characteristics, bandwidth, and/or gain. For example, most slotted dual-band patch antennas are either linearly polarized or have poor circular polarization performance at the desired dual frequency bands.
An exemplary environment in which micro-strip antennas can operate is that of satellite communications and satellite navigation. In satellite communications, for example, circular polarization is preferred over other types of polarization. Circular polarization, among other things, factors into account the movement of the satellite with respect to the Earth. Circular polarization also takes into account other anomalies, such as Faraday rotation and depolarization caused by precipitation particles such as rain and ice in the atmosphere.
Global positioning system (GPS) satellites, used in navigation for example, require circular polarization. In fact, optimizing the performance of GPS requires not only circular polarization, but requires that the corresponding circular polarization electromagnetic components, be relatively pure. Also, slots that are traditionally used to provide dual-band operation from conventional micro-strip antennas are generally designed as narrow-band filters.
GPS and other satellite communication systems, however, require much wider bandwidths for optimal performance. In fact current military GPS navigation systems require GPS antennas to operate in two separate frequency bands centered at 1575 and 1227 MHz in order to provide better precision accuracy in range by allowing correction for errors introduced by the ionosphere.
GPS navigation systems are also being modernized. To accommodate military M code signals, future GPS systems starting in 2005, will require at least 24 megahertz (MHz) bandwidth antennas. In addition a third frequency band called L5, operating at a center frequency of 1176 MHz will also be added to the current frequency bands, L1 (center frequency of 1575 MHz) and L2 (center frequency of 1227 MHz).
Civilian navigation systems, including the FAA, will rely mainly on the L1 and L5 frequency bands whereas U.S. military systems will rely primarily on L1 and L2 frequency bands, both of which will have an enhanced bandwidth of 24 MHz to also accommodate the new military M Code signals.
The operating bandwidths of conventional micro-strip patch antennas are much too narrow to satisfy these future GPS multiband frequency requirements and will place an additional emphasis on the need for newer microstrip antenna designs capable of providing dual band or even triple band operation with the desired bandwidth, gain and circular polarization characteristics to meet operational needs in both civilian and military GPS navigational systems.
Additionally, the European Union will also be launching in 2005 their own version of a GPS navigation system called “Galileo”. The Galileo frequencies are 1575 and 1176 MHz—which is the same as for the modernized U.S. GPS system as well as two new frequencies centered at 1207 MHz and 1279 MHz. Hence there will be strong commercial interest in developing broadband dual-band or even triple-band GPS antennas that cover all or some of both the U.S. GPS and the European Galileo navigation systems to be launched starting in 2005.
Some of the other slotted bandwidth enhancement designs restrict radio frequency (RF) current flow on the surface of the patch to narrow areas. This current restriction generally decreases the antenna gain at the second, or resonant, frequency achieved by inserting the slot. The radiation pattern at the resonant frequency is also asymmetric and distorted because of this uneven current flow. This asymmetry further decreases performance.
What is needed, therefore, is a compact and light-weight micro-strip patch antenna that is capable of operating in at least two different frequency bands. It is desirable that this dual-band patch antenna be circularly polarized and located on top of a single dielectric substrate layer or a plurality of dielectric substrate layers. It is also desirable that this patch antenna provide good gain over a large portion of the upper hemisphere to acquire signals from multiple satellites over a wide range of elevation angles.
What is also needed is a dual band co-planar antenna having a broad enough bandwidth to cover both the U.S. GPS and Galileo frequencies at 1176 and 1575 MHz as well as the additional Galileo frequency centered at 1207 MHz.